This invention is directed to an improved electron gun support structure for guns used in television cathode ray tubes.
Electron guns for television cathode ray tubes generate one or more beams of electrons by cathodic thermionic emission. The resulting beams are formed and shaped by a tandem succession of electrodes spaced along the central axis of the gun. The electrodes are commonly supported by a plurality of elongated, axially oriented structural beads, or "pillars", of glass. The electrodes cause the beams to be focused on electro-luminescent phosphors located on the inside of the faceplate of the cathode ray tube. In the context of today's television systems, electron guns for television cathode ray tubes may be the single-beam type for black-and-white or index-type color television picture tubes, or, they may be of the three-beam, in-line type or the three-beam, delta-type for color picture tubes.
It is essential to the proper performance of an electron gun that the gun, as installed in the neck of the cathode ray tube envelope, be in proper alignment with the center axis of the tube, which in turn perpendicularly intersects the faceplate, typically rectangular, at the intersection of the major and minor axes thereof. Also, the gun must be installed in rotational alignment with the (horizontal) planes in which the beams are scanned. And when once installed, the gun must remain in proper alignment and be immune to the dynamic forces resulting from thermal cycling associated with sporadic tube operations, as well as to forces produced by shock and vibration. Any gun misalignment is symptomized in color tubes, depending upon the type of tube, by effects such as color impurity and color fringing, lack of resolution, improper gray scale, and lack of operating stability.
Typically, in the process of cathode ray tube manufacture, the electron gun is installed in the glass cathode ray tube by inserting it into the neck of the tube, which is commonly flared. (The flared section is later removed). The forward end of the gun is centered in the neck by means of a plurality of contact or centering springs extending forwardly and outwardly from the upper or forwardmost electrode of the gun; that is, the electrode nearest the screen. This electrode is commonly termed the "convergence cup", or "support cup". The opposite end of the gun, that is, the rear end of the gun, is conventionally supported and centered in the neck of the tube by attachment of electrode electrical connections to a plurality of electrically conductive lead-in pins extending through a glass base of the tube. Joining of the glass base to the glass neck of the tube is accomplished by melting and joining the contiguous glass surfaces to provide a mechanical attachment and an air-tight seal. Thus, the electron gun is commonly supported within the neck of the cathode ray tube in two areas -- at the forward end by the centering springs, and at the rear end by its attachment to the lead-in pins that extend through the glass base.
One deficiency of the prior art support system lies in the method commonly used to attach the electron gun assembly to the glass base, briefly alluded to above. The electron gun is comprised of a succession of components such as one or more cathodes, beam control electrodes, the beam-forming and beamfocusing electrodes of the main focus lens, and the support cup. As is well known in the art, these components, which collectively constitute the gun assembly, are mechanically fixed in proper relationship with each other in a rigid, coherent unit by a plurality of glass beads, or "pillars", that extend in a direction parallel to the center axis of the gun. In common practice, this gun assembly is in turn attached to the lead-in pins in the tube base by means of a plurality of stiff electrical wires which transmit operating power to cathode filaments, and electrical potentials to the beam-controlling, beam-forming and beam-focusing electrodes. (An example of the use of such wires is shown by FIG. 1 of U.S. Pat. No. 3,462,634). Each of these conductive wires must be cut to proper length and pre-formed into the proper configuration for interconnecting a lead-in pin and a gun electrode. While these wires are satisfactory as electrical conductors, they leave much to be desired in terms of providing firm mechanical support for the rear end of the electron gun. The resulting grid of supporting wires is inherently physically weak and vulnerable not only to shock and vibration, but also to distortion and twisting as a result of thermal cycling influences due to their propinquity to the cathode heater filaments. The wires may also flex in response to "memory" of their shape prior to forming, especially as aggravated by thermal cycling.
So the lower end of the gun is relatively unsupported and unstable, and deflection of the supporting electrical wires can cause abaxial and/or rotational misalignment, resulting too often in marked degradation of electron gun performance.
There is another drawback to the recited structure that relates to problems and costs of manufacture. The pre-formed wires must be installed by hand, with electrical attachment made by welding. Such labor-intensive methods are costly and can result in many rejects. Yields are further decreased by the fact that lead-in pins often have to be bent by pliers for proper mating with the pre-formed wires, with the result that the lead-in pin fillet may crack and the entire tube base will have to be discarded.
With regard to the prior art in electron gun mounting systems, it is known that a type of electrode support was provided in early radio receiving vacuum tubes that took the form of one or more internal mica discs positioned inside the tube envelope and near the base of the tube through which the lead-in pins passed. The purpose of these discs was to keep the electrical leads to the electrodes physically separated so as to prevent short circuits. No physical support of tube electrodes is ascribed to these mica spacers. No specific related prior art publications describing this application are known.
U.S. Pat. No. 3,716,739 discloses an annular electrically conductive support and shielding means for a compact electron gun. The structure is provided to avoid the use of the conventional glass beads. The support and shielding means is placed at the anode potential.